Composition for inhibiting calcium salt scale

ABSTRACT

Compositions and method of improving inhibition of calcium salt scale formation under the conditions found in chemical pulp processes in which an effective amount of selected phosphonates or phosphonate blends is admixed with the black liquor composition recovered from the digester in a chemical pulping process. The compositions and method are especially well suited for use in the Kraft pulping process.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a nonprovisional application which claims thepriority of prior provisional application Ser. No. 60/296,356, entitled“Method for Inhibiting Calcium Salt Scale,” filed Jun. 6, 2001, which ishereby incorporated by reference into this application.

FIELD OF THE INVENTION

This invention relates to compositions and methods for inhibiting scaleformation in aqueous alkaline systems of chemical pulping processes.This invention further relates to compositions and methods forinhibiting formation, deposition and adherence of calcium salt scaledeposits in chemical pulping process equipment. More particularly, thisinventions relates to compositions and methods for inhibiting formation,deposition and adherence of calcium salt scale deposits in the blackliquor recovery area of a chemical pulping process.

BACKGROUND OF THE INVENTION

Paper is widely used worldwide in commerce and in homes and has avariety of uses. Pulp making is thus carried out on a large industrialscale worldwide to produce sufficient quantities of paper. Accordinglyit is highly desirable that such pulp making operations be carried outin a cost effective, efficient operation with minimum manufacturingequipment downtime and minimum periods of reduced pulp making processequipment efficiency.

The basic steps in industrial pulp making are to convert plant fiberinto chips, convert chips into pulp, (optionally) bleach the pulp, washthe pulp, and transform the pulp into suitable paper which can be usedin paper products such as writing paper, newsprint and paper fordocuments.

Typically, several chemical pulping processes are used in industrialpulp making operations. Well known industrial alkaline chemical pulpingprocesses include the Kraft (or sulfate), soda and alkaline sulfiteprocesses. The Kraft process makes the strongest fibers of any pulpproducing process and is the most commonly used pulp making process inpart due to its efficient recovery process for the cooking chemicals.While the present invention has applicability to any of the abovealkaline chemical pulping processes, it is particularly useful with theKraft process and, as such, the Kraft process is described in moredetail below.

Initially, suitable trees are harvested, debarked and then chipped intosuitable size flakes or chips. These wood chips are sorted with thesmall and the large chips being removed. The remaining suitable woodchips are then charged to a digester (which is a vessel or tank forholding the chips and an aqueous digesting composition, such tanks canbe designed for either batch or continuous operation).

Illustratively, in a batch type digester, wood chips and a mixture of“weak black liquor,” the spent liquor from a previous digester cook, and“white liquor,” a solution of sodium hydroxide and sodium sulfide, thatis either fresh or from the chemical recovery plant, is pumped into thedigester. In the cooking process lignin, which binds the wood fibertogether, is dissolved in the white liquor forming pulp and blackliquor.

The digester is sealed and the digester composition is heated to asuitable cook temperature under high pressure. After an allotted cookingtime at a particular temperature and pressure (H-factor) in thedigester, the digester contents (pulp and black liquor) are transferredto a holding tank. The pulp in the holding tank is transferred to brownstock washers while the liquid (black liquor formed in the digester) issent to the black liquor recovery area, i.e. black liquor evaporators.The black liquor is evaporated to a high solids content, usually 60-80%solids, using a multiple effect evaporator, for example. The higher thesolids content, the more difficult it is to pump the black liquor andthe more scale problems the pulp mill will have. One of the mosttroublesome is calcium carbonate scale which forms in various areas ofthe pulp mill, including the digester, the black liquor evaporator area,and the brown stock washing area.

Most commercial paper mills use multiple effect evaporators (MEE) as theblack liquor evaporators. These evaporators generally range from four toeight effects in length. Generally, undesirable calcium carbonatescaling occurs in only one or two effects. Currently, most mills do notuse any scale inhibitor but rather contend with the scale problem byshutting down the black liquor evaporator section and washing out thecalcium carbonate scale with hot acid, i.e. acid cleaning. This hot acidboil out adversely affects papermill production and is a concern becausethe acid used is corrosive to mill piping and equipment.

The Kraft cook is highly alkaline, usually having a pH of 10 to 14, moreparticularly 12 to 14. The digester composition contains a large amountof sodium sulfide, which is used as an accelerant to increase thedelignification rate of the cook. This works to release the lignin inthe wood chips and thus the cellulose becomes available as pulp.

The combination of operating conditions in the Kraft process isconducive to scale formation and deposition and increases the propensityof the calcium carbonate scale to form, deposit and adhere to metallicand other surfaces within which it comes in contact. Under such processconditions, calcium present in the water and leached from the wood inthe Kraft process can react with carbonate and produce rather rapidscaling with the deposition of calcium carbonate scale. Such scale isfrequently deposited in the black liquor evaporator, the digester, andassociated piping, heat exchangers, etc., all of which have surfaces onwhich the calcium carbonate can deposit and adhere. Such depositionbuilds up over time and can result in undesirable premature shutdownsdownstream on the pulp making manufacturing line to remove scaledeposits by hot acid washing.

Several patents and a technical article disclose problems of scaling. In“An Effective Sequestrant For Use In Controlling Digester Scale,” R. H.Windhager, Paper Trade Journal, pp. 42-44, Nov. 5, 1973, the use ofsmall quantities of mono-aminomethylene phosphonic acid (ATMP) as acalcium carbonate scale inhibitor in a digester to inhibit scaledeposition from the digester cooking liquor is disclosed.

U.S. Pat. No. 4,799,995 (issued to Druce K. Crump et al. on Jan. 24,1989) discloses that inhibition of calcium scale under conditions foundin pulp digesters has been accomplished by employing mixtures ofpolyamino(polyalkylenephosphonic) acids with non-ionic surfactants addedto the pulp liquor. This U.S. patent also discloses that phosphonatessuch as nitrilotris(methylenephosphonic acid) (“NTMP” or “ATMP”),1-hydroxyethane-1,1-diphosphonic acid (“HEDP”) and sodium1-hydroxyethane-1,1-diphosphonate (“NaHEDP”) are said to have beencommonly used to control scale. However, the '995 patent discloses thatthe use of HEDP in black liquor actually promoted scale and use ofdiethylenetriamine penta(methylenephosphonic acid) (“DTPMP”) in blackliquor without the presence of a nonionic surfactant resulted in onlylimited scale reduction. While the '995 patent discloses the use ofnonionic surfactants to improve scale reduction, it is preferred toavoid the use of surfactants in chemical pulp processes, particularly inthe digester. The compositions of the present invention when added to analkaline chemical pulp process digester are effective at inhibitingcalcium salt scale in chemical pulp processes without the need for anonionic surfactant.

Canadian Patent No. 1,069,800 (Philip S. Davis et al., Jan. 15, 1980)discloses the addition of blends of organophosphonates, e.g.1-hydroxyethylidene-1,1-diphosphonic acid (HEDP), with amino-organophosphonates, e.g. amino tri(methylenephosphonic acid) (AMP),ethylenediamine tetra(methylenephosphonic acid) (EDTPA), andhexamethylenediamine tetra(methylenephosphonic acid) (HMDTA), to blackliquor to reduce calcium carbonate scale in a black liquor evaporatorsystem at a pH above 9. This patent also discloses that use ofindividual (single) phosphonates, instead of the disclosed blends, werenot effective at a pH above 9 to inhibit calcium carbonatecrystallization.

U.S. Pat. No. 4,851,490 (issued to Fu Chen et al. on Jul. 25, 1989)discloses water soluble polymers containing hydroxyalkyleneaminoalkylenephosphonate functions which are said to have utility as deposit controlagents effective in a number of water systems such as cooling, boilers,conversion coating, paper and pulp processing and gas scrubbing.

U.S. Pat. No. 5,534,157 (issued to Craig D. Iman et al. on Jul. 9, 1996)discloses a method for inhibiting the formation, deposition andadherency of scale-forming salts in process waters at high pH utilizingpolyether polyamine methylene phosphonates. At column 4, lines 35-51thereof, this U.S. patent discloses that inhibitors such as HEDP andATMP are useless as scale inhibitors at alkaline pH conditions.

U.S. Pat. No. 5,562,830 (issued to Davor F. Zidovec et al. on Oct. 8,1996) discloses a method of inhibiting corrosion and scale formation anddeposition in aqueous systems by adding a combination of apolyepoxysuccinic acid or salts thereof and a phosphonocarboxylic acidor salts thereof.

U.S. Pat. No. 5,552,018 (issued to Johan Devenyns on Sep. 3, 1996)discloses a process in which a peroxyacid is employed to improve theselectivity of the delignification of a chemical paper pulp that hasalready undergone a delignifying treatment in the presence of chemicalreagents, i.e. a Kraft cook. Phosphonates are disclosed as stabilizersin this process.

Despite the aforementioned patents and technical article, enhancedmethods and compositions for inhibiting the formation, deposition andadherence of scale to metallic surfaces particularly in commercialchemical pulp processing equipment is highly desired.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a composition forinhibiting the formation, deposition and adherence of calcium salt scaleto metallic and other surfaces in the equipment, vessels and/or pipingof a chemical pulp process facility. It is another object of thisinvention to provide a method for inhibiting the formation, depositionand adherence of calcium salt scale to surfaces in the equipment,vessels and/or piping of a chemical pulp process facility.

These and other objects are achieved in the invention which is describedin more nonlimiting detail hereinafter.

According to the invention, a scale inhibiting composition forinhibiting calcium salt scale formation in alkaline aqueous mixtures ofchemical pulping processes is provided, wherein the composition is addedto the black liquor of the chemical pulping process, the compositioncomprising an effective scale inhibiting amount of at least onephosphonate selected from compounds having the formula:M₂O₃P—CH₂—N(R¹)—(CH₂)_(m)—N(R²)—CH₂PO₃M₂  (I),compounds having the formula:R³—C(OH)(PO₃M₂)₂  (II),compounds having the formula:N—(CH₂PO₃M₂)₃  (III),phosphonates having the formula:

amine oxides of phosphonates of formulas (I) and (III), or mixturesthereof; wherein M is independently selected from hydrogen, alkalimetal, alkaline earth metal or ammonium, R¹ and R² are independentlyselected from CH₂PO₃M₂ or (CH₂)_(n)—N—(CH₂PO₃M₂)₂, m is 2 or 3, n is 2or 3, and R³ is an alkyl group having 1 to 17 carbon atoms and R³ isoptionally branched and optionally unsaturated; with the provisos that:

-   -   (a) the phosphonate is not a blend of a phosphonate of        formula (II) with a phosphonate of formula (III),    -   (b) the phosphonate is not a blend of a phosphonate of        formula (II) with a phosphonate of the formula        (M₂O₃P—CH₂)₂—N—(CH₂)₂—N—(CH₂PO₃M₂)₂,    -   (c) when the phosphonate is selected from phosphonates of        formula (III), phosphonates of the formula        (M₂O₃P—CH₂)₂—N—(CH₂)₂—N—(CH₂PO₃M₂)₂, or phosphonates of the        formula (M₂O₃P—CH₂)₂—N—(CH₂)₂—N—(CH₂PO₃M₂)—(CH₂)₂—N—(CH₂PO₃M₂)₂,        the scale inhibiting composition does not contain a nonionic        surfactant,    -   (d) when the phosphonate is selected from phosphonates of        formula (III), or phosphonates of the formula        (M₂O₃P—CH₂)₂—N—(CH₂)₂—N—(CH₂PO₃M₂)₂—N—(CH₂PO₃M₂)₂, the amount of        the phosphonate on an active acid basis is greater than 25 ppm        based on the weight of black liquor recovered from the digester,        and    -   (e) when the phosphonate is selected from the phosphonates of        formula (IV), the amount of the phosphonate on an active acid        basis is greater than 20 ppm based on the weight of black liquor        recovered from the digester.

Further according to the invention, a method for inhibiting calcium saltscale formation in chemical pulping processes is provided comprisingadmixing an effective scale inhibiting amount of the above compositionwith the black liquor recovered from the digester of the chemicalpulping process.

Still further according to the invention, a method for inhibitingcalcium salt scale formation in an aqueous system in a selected alkalinechemical pulping process is provided comprising: (a) determining thecalcium salt scale inhibition profiles of phosphonate concentration andprocess temperature as a function of time for phosphonate compositionsadmixed with the black liquor composition recovered from the digester ofthe chemical pulping process, (b) identifying the calcium salt scaleinhibition capability required by the selected chemical pulping processbased on the process operating conditions of time, temperature andpressure, and the black liquor composition, (c) selecting theappropriate phosphonate composition and phosphonate use concentration toeffectively inhibit calcium salt scale formation in the selectedchemical pulping process when the phosphonate is admixed with the blackliquor composition recovered from the digester of the selected alkalinechemical pulping process based on steps (a) and (b), and (d) admixingthe selected phosphonate composition with the black liquor compositionin the selected alkaline chemical pulping process during the blackliquor recovery stage of the chemical pulping process; wherein theselected phosphonate composition is as defined above.

Still further according to the invention, a method for inhibitingcalcium salt scale formation in an aqueous system in a selected alkalinechemical pulping process is provided comprising: (a) identifying thecalcium salt scale inhibition capability required by the selectedchemical pulping process based on the process operating conditions oftime, temperature and pressure, and the black liquor composition, (b)selecting the appropriate phosphonate composition and phosphonate useconcentration to effectively inhibit calcium salt scale formation in theselected alkaline chemical pulping process when the phosphonate isadmixed with the black liquor composition recovered from the digester inthe selected alkaline chemical pulping process based on step (a) and thecalcium salt scale inhibition profiles of phosphonate concentration andprocess temperature as a function of time for phosphonate compositionsadmixed with the black liquor composition recovered from the digester ina chemical pulping process, and (c) admixing the selected phosphonatecomposition with the black liquor composition recovered from thedigester in the selected alkaline chemical pulping process during thedigestion stage of the chemical pulping process; wherein the selectedphosphonate composition is as defined above.

DETAILED DESCRIPTION OF THE DRAWINGS NOT APPLICABLE DETAILED DESCRIPTIONOF THE INVENTION

A first embodiment of the invention relates to a scale inhibitingcomposition for inhibiting calcium salt scale formation in alkalineaqueous mixtures of chemical pulping processes, wherein the compositionis added to the black liquor of the chemical pulping process, thecomposition comprising an effective scale inhibiting amount of at leastone phosphonate selected from compounds having the formula:M₂O₃P—CH₂—N(R¹)—(CH₂)_(m)—N(R²)—CH₂PO₃M₂  (I),compounds having the formula:R³—C(OH)(PO₃M₂)₂  (II),compounds having the formula:N—(CH₂PO₃M₂)₃  (III),phosphonates having the formula:

amine oxides of phosphonates of formulas (I) and (III), or mixturesthereof; wherein M is independently selected from hydrogen, alkalimetal, alkaline earth metal or ammonium, R¹ and R² can be the same ordifferent and are independently selected from —CH₂PO₃M₂ or—(CH₂)_(n)—N—(CH₂PO₃M₂)₂, m is 2 or 3, n is 2 or 3, and R³ is an alkylgroup having 1 to 17 carbon atoms and R³ is optionally branched andoptionally unsaturated; with the provisos that:

-   -   (a) the phosphonate is not a blend of a phosphonate of        formula (II) with a phosphonate of formula (III),    -   (b) the phosphonate is not a blend of a phosphonate of        formula (II) with a phosphonate of the formula        (M₂O₃P—CH₂)₂—N—(CH₂)₂—N—(CH₂PO₃M₂)₂,    -   (c) when the phosphonate is selected from phosphonates of        formula (III), phosphonates of the formula        (M₂O₃P—CH₂)₂—N—(CH₂)₂—N—(CH₂PO₃M₂)₂, or phosphonates of the        formula (M₂O₃P—CH₂)₂—N—(CH₂)₂—N—(CH₂PO₃M₂)—(CH₂)₂—N—(CH₂PO₃M₂)₂,        the scale inhibiting composition does not contain a nonionic        surfactant,    -   (d) when the phosphonate is selected from phosphonates of        formula (III), or phosphonates of the formula        (M₂O₃P—CH₂)₂—N—(CH₂)₂—N—(CH₂PO₃M₂)—(CH₂)₂—N—(CH₂PO₃M₂)₂, the        amount of the phosphonate on an active acid basis is greater        than 25 ppm based on the weight of black liquor recovered from        the digester, and    -   (e) when the phosphonate is selected from the phosphonates of        formula (IV), the amount of the phosphonate on an active acid        basis is greater than 20 ppm based on the weight of black liquor        recovered from the digester.

In the phosphonates of the invention, M is preferably hydrogen or alkalimetal, and the alkali metal is preferably sodium or potassium, R³ ispreferably an alkyl group having 1 to 5 carbon atoms, more preferablymethyl, and m is preferably 2.

The scale inhibiting compositions of the invention include, but are notlimited to, at least one phosphonate of formula (I), at least onephosphonate of formula (II), at least one phosphonate of formula (III),at least one phosphonate of formula (IV), at least one amine oxide of aphosphonate of formulas (I) or (III), a mixture of at least twophosphonates of formula (I), a mixture of at least one phosphonate offormula (I) and at least one phosphonate of formula (II), formula (III)or formula (IV), a mixture of at least two phosphonates of formula (II),or a mixture of at least one amine oxide of a phosphonate of formulas(I) or (III) and at least one phosphonate of formulas (I) or (III).Preferably, the scale inhibiting composition of the invention is atleast one phosphonate of formula (I), a mixture of at least twophosphonates of formula (I), a mixture of at least one phosphonate offormula (I) and at least one phosphonate of formula (II) or formula(III), or a mixture of at least one phosphonate of formula (II) and atleast one phosphonate of formula (IV).

Examples of suitable phosphonates include, but are not limited to, thephosphonates in Table 1 below. Table 1 below provides formulas forrepresentative phosphonates of formulas (I), (II) and (III). Thephosphonates in Table 1 are available from Solutia Inc., 575 MaryvilleCentre Drive, St. Louis, Mo. under the trademark Dequest® phosphonatesand are identified by their Dequest® phosphonate product number.

TABLE 1 Dequest Product No. Formula R¹ R² m n R³ M 2000 III — — — — — 6H 2006 III — — — — — 5 Na, 1 H 2010 II — — — — CH₃ 4 H 2016 II — — — —CH₃ 4 Na 2041 I CH₂PO₃M₂ CH₂PO₃M₂ 2 — — 8 H 2046 I CH₂PO₃M₂ CH₂PO₃M₂ 2 —— 3H, 5 Na 2060 I CH₂PO₃M₂ (CH₂)_(n)N(CH₂PO₃M₂)₂ 2 2 — 10 H 2066 ICH₂PO₃M₂ (CH₂)_(n)N(CH₂PO₃M₂)₂ 2 2 — 7 Na, 3 H 7000 IV — — — — — 5 H

The formulas and corresponding names of the Dequest phosphonates listedin Table 1 are shown below.

-   -   Dequest 2000—amino-tris(methylenephosphonic acid) N(CH₂PO₃H₂)₃    -   Dequest 2006—sodium salt of amino-tris(methylenephosphonic acid)        Na₅H[N(CH₂PO₃)₃]    -   Dequest 2010—1-hydroxyethylidene (1,1-diphosphonic acid)        CH₃C(OH)(PO₃H₂)₂    -   Dequest 2016—sodium salt of 1-hydroxyethylidene        (1,1-diphosphonic acid) Na₄[CH₃C(OH)(PO₃)₂]    -   Dequest 2041—ethylenediamine tetra(methylenephosphonic acid)        H₈[(O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃)₂]    -   Dequest 2046—ethylenediamine tetra(methylenephosphonic acid),        pentasodium salt Na₅H₃[(O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃)₂]    -   Dequest 2060—diethylenetriamine-penta(methylenephosphonic acid)        (H₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃H₂)CH₂CH₂N(CH₂PO₃H₂)₂    -   Dequest 2066—sodium salt of        diethylenetriamine-penta(methylenephosphonic acid)        Na₇H₃[(O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃)CH₂CH₂N(CH₂PO₃)₂]    -   Dequest 7000—2-phosphonobutane-1,2,4-tricarboxylic acid

Another preferred phosphonate of formula (I) is the compoundN,N′-bis(3-aminopropyl)ethylenediamine-hexa(methylenephosphonic acid),or a salt thereof wherein the salt is sodium, potassium, ammonium, andthe like. When the compound is the sodium salt, the compound has theformulaNa_(x)H_(y)[(O₃PCH₂)₂NCH₂CH₂CH₂N(CH₂PO₃)CH₂CH₂N(CH₂PO₃)CH₂CH₂CH₂N—(CH₂PO₃)₂];wherein x+y is 12, and is designated herein as 4NHMP. This compound canbe prepared according to the procedure disclosed in Example 1 of U.S.Pat. No. 5,261,491, which is herein incorporated by reference.

The preferred phosphonates of formula (I) are

-   -   (M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)₂,    -   (M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)₂or    -   (M₂O₃PCH₂)₂NCH₂CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂CH₂N—    -   (CH₂PO₃M₂)₂, more preferably (M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)₂ or    -   (M₂O₃PCH₂)₂NCH₂CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂CH₂N—(CH₂PO₃M₂)₂,        and most preferably    -   (M₂O₃PCH₂)₂NCH₂CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂CH₂N—(CH₂PO₃M₂)₂.

The preferred phosphonate of formula (II) is H₃C—C(OH)(PO₃M₂)₂, withH₃C—C(OH)(PO₃Na₂)₂being more preferred.

Blends of at least two phosphonates independently selected from thephosphonates of formulas (I), (II), (III), (IV) and amine oxides of thephosphonates of formulas (I) and (III) may be used according to theinvention. It is currently preferred to use a blend of two phosphonates,with a blend of a phosphonate of formula (I) with either a phosphonateof formula (I), formula (II), formula (III) or formula (IV) being morepreferred, and a blend of two phosphonates of formula (I) being mostpreferred. The composition of the blends can vary over a wide range withthe percentage of each component ranging broadly from 1 to 99 wt. %,provided each phosphonate is present in an amount of at least about 1wt. %. Preferably, each phosphonate is present in an amount of at leastabout 10 wt. %. In the case of a two component blend, each phosphonateis present preferably in an amount of about 10 to about 90 wt. %, andmore preferably in an amount of about 20 to about 80 wt. %.

The preferred blends for use in the invention are blends of aphosphonate selected fromN,N′-bis(3-aminopropyl)ethylenediamine-hexa(methylenephosphonic acid),diethylenetriamine-penta(methylenephosphonic acid), or salts thereofwith a phosphonate selected from the phosphonates of formulas (I), (II),(III) or (IV), or a phosphonate selected from ethylenediaminetetra(methylenephosphonic acid), or salts thereof with a phosphonateselected from the phosphonates of formulas (I) or (III). More preferredare blends of phosphonates selected fromN,N′-bis(3-aminopropyl)ethylenediamine-hexa(methylenephosphonic acid),ethylenediamine tetra(methylenephosphonic acid),diethylenetriamine-penta(methylenephosphonic acid), or salts thereofwith another phosphonate selected from the phosphonates of formula (I)and blends ofN,N′-bis(3-aminopropyl)ethylenediamine-hexa(methylenephosphonic acid),diethylenetriamine-penta(methylenephosphonic acid) or salts thereof witha phosphonate selected from the phosphonates of formula (II).

An effective amount of phosphonate or mixtures of phosphonates isemployed in making and using the scale inhibiting composition of thisinvention. That effective amount depends on the particularphosphonate(s) employed in practicing this invention and other factorsincluding, but not limited to, the digester composition, the operatingconditions (i.e. H-factor) of the digester, the black liquorcomposition, and operating conditions in the brown stock washing areaand black liquor recovery area, as well as other factors and conditionsknown to those of ordinary skill in the art. Selection of the effectiveamount of phosphonate will be readily apparent to one of ordinary skillin the art after reading this specification.

When the scale inhibiting composition of the invention is at least onephosphonate of formula (I), the phosphonate(s) and the effective scaleinhibiting amount of each is as follows.

As used herein, the ppm usage level of scale inhibitor is based on theweight of total liquor charged with the liquor assumed to have a densityof 1 g/mL.

When the phosphonate is (M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)₂, the effectiveamount of the phosphonate on an active acid basis is about 10 ppm toabout 1000 ppm, preferably about 20 ppm to about 500 ppm, and morepreferably about 30 to about 500 ppm, based on the weight of blackliquor recovered from the digester.

When the phosphonate is (M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)₂,the effective amount of phosphonate on an active acid basis is about 30ppm to about 1000 ppm, preferably about 40 ppm to about 500 ppm, basedon the weight of black liquor recovered from the digester.

When the phosphonate is(M₂O₃PCH₂)₂NCH₂CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃)M₂)CH₂CH₂CH₂N—(CH₂PO₃M₂)₂,the effective amount of phosphonate on an active acid basis is about 10ppm to about 1000 ppm, preferably about 20 ppm to about 500 ppm, basedon the weight of black liquor recovered from the digester.

When the scale inhibiting composition of the invention is at least onephosphonate of formula (II), the phosphonate is preferablyCH₃C(OH)(PO₃M₂)₂ and the effective scale inhibiting amount ofphosphonate on an active acid basis is about 20 ppm to about 200 ppm,preferably about 30 ppm to about 100 ppm, based on the weight of blackliquor recovered from the digester.

When the scale inhibiting composition of the invention is at least onephosphonate of formula (III), the effective scale inhibiting amount ofphosphonate on an active acid basis is about 50 to about 1000 ppm,preferably about 80 to about 500 ppm, based on the weight of blackliquor recovered from the digester.

When the scale inhibiting composition of the invention is at least onephosphonate of formula (IV), the effective scale inhibiting amount ofphosphonate on an active acid basis is about 50 to about 500 ppm,preferably about 100 to about 200 ppm, based on the weight of blackliquor recovered from the digester.

When the scale inhibiting composition of the invention is at least oneamine oxide of a phosphonate of formula (I) or formula (III), theeffective scale inhibiting amount of amine oxide is the amount on anactive acid basis that is equivalent to the effective amount of thecorresponding phosphonate of formula (I) or formula (III).

When the scale inhibiting composition of the invention is a mixture ofat least two phosphonates of formula (I) of a mixture of at least onephosphonate of formula (I) and at least one phosphonate of formula(III), the phosphonate(s) and the effective scale inhibiting amount ofeach is as follows.

When the first phosphonate is(M₂O₃PCH₂)₂NCH₂CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂CH₂—N(CH₂PO₃M₂)₂,the second phosphonate is preferably selected from N(CH₂PO₃M₂)₃,(M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)₂, or(M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)₂. When the secondphosphonate is N(CH₂PO₃M₂)₃, the amount of the mixture on an active acidbasis is about 10 ppm to about 1000 ppm, preferably about 200 ppm toabout 500 ppm, based on the weight of black liquor recovered from thedigester. When the second phosphonate is (M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)₂,the amount of the mixture on an active acid basis is about 20 ppm toabout 1000 ppm, preferably about 30 ppm to about 500 ppm, based on theweight of black liquor recovered from the digester. When the secondphosphonate is (M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)₂, theamount of the mixture on an active acid basis is about 10 ppm to about1000 ppm, preferably about 30 ppm to about 500 ppm, based on the weightof black liquor recovered from the digester.

When the first phosphonate is (M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)₂, the secondphosphonate is preferably selected from(M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)₂. When the secondphosphonate is (M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)₂, theamount of the mixture on an active acid basis is about 20 ppm to about1000 ppm, preferably about 40 ppm to about 500 ppm, based on the weightof black liquor recovered from the digester. When the second phosphonateis N(CH₂PO₃M₂)₃, the amount of the mixture on an active acid basis isabout 30 ppm to about 1000 ppm, preferably about 50 ppm to about 500ppm, based on the weight of black liquor recovered from the digester.

When the first phosphonate is(M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)₂, and the secondphosphonate is N(CH₂PO₃M₂)₃, the amount of the mixture on an active acidbasis is about 50 ppm to about 1000 ppm, preferably about 150 ppm toabout 500 ppm, based on the weight of black liquor recovered from thedigester.

The preferred blends of at least two phosphonates of formula (I) or atleast one phosphonate of formula (I) and at least one phosphonate offormula (III) are blends of(M₂O₃PCH₂)₂NCH₂CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂CH₂—N(CH₂PO₃M₂)₂with N(CH₂PO₃M₂)₃, (M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)₂, or(M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)₂, or blends of(M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)₂ with(M₂O₃PCH₂)₂NCH₂CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂CH₂N—(CH₂PO₃M₂)₂,(M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)CH₂PO₃M₂)₂, or N(CH₂PO₃M₂)₃.

The most preferred blends of at least two phosphonates of formula (I) orat least one phosphonate of formula (I) and at least one phosphonate offormula (III) are blends of(M₂O₃PCH₂)₂NCH₂CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)—CH₂CH₂CH₂N(CH₂PO₃M₂)₂with N(CH₂PO₃M₂)₃, (M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)₂, or(M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)₂.

When the scale inhibiting composition of the invention is a mixture ofat least one phosphonate of formula (I) and at least one phosphonate offormula (II), the phosphonate(s) and the effective scale inhibitingamount of each is as follows.

Preferred blends are mixtures of a first phosphonate selected from(M₂O₃PCH₂)₂NCH₂CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂CH₂N—(CH₂PO₃M₂)₂or (M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)₂, and a secondphosphonate selected from CH₃C(OH)(PO₃M₂)₂.

When the first phosphonate is selected from(M₂O₃PCH₂)₂NCH₂CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂CH₂N—(CH₂PO₃M₂)₂,the amount of the mixture on an active acid basis is about 10 ppm toabout 500 ppm, preferably about 30 ppm to about 150 ppm, based on theweight of black liquor recovered from the digester. When the firstphosphonate is (M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)₂, theamount of the mixture on an active acid basis is about 30 ppm to about1000 ppm, preferably about 50 ppm to about 200 ppm, based on the weightof black liquor recovered from the digester.

The most preferred blends of at least one phosphonate of formula (I) andat least one phosphonate of formula (II) are blends of(M₂O₃PCH₂)₂NCH₂CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂CH₂N—(CH₂PO₃M₂)₂and CH₃C(OH)(PO₃M₂)₂.

A second embodiment of the invention relates to a method for inhibitingcalcium salt scale formation in alkaline chemical pulping processescomprising adding an effective scale inhibiting amount of at least onephosphonate to the black liquor of the chemical pulping process, thecomposition comprising at least one phosphonate selected from compoundshaving the formula:M₂O₃P—CH₂—N(R¹)—(CH₂)_(m)—N(R²)—CH₂PO₃M₂  (I),compounds having the formula:R³—C(OH)(PO₃M₂)₂  (II),compounds having the formula:N—(CH₂PO₃M₂)₃  (III),phosphonates having the formula:

amine oxides of phosphonates of formulas (I) and (III), or mixturesthereof; wherein M is independently selected from hydrogen, alkalimetal, alkaline earth metal or ammonium, R¹ and R² are independentlyselected from —CH₂PO₃M₂ or —(CH₂)_(n)—N—(CH₂PO₃M₂)₂, m is 2 or 3, n is 2or 3, and R³ is an alkyl group having 1 to 17 carbon atoms and R³ isoptionally branched and optionally unsaturated; with the provisos that:

-   -   (a) the phosphonate is not a blend of a phosphonate of        formula (II) with a phosphonate of formula (III),    -   (b) the phosphonate is not a blend of a phosphonate of        formula (II) with a phosphonate of the formula        (M₂O₃P—CH₂)₂—N—(CH₂)₂—N—(CH₂PO₃M₂)₂,    -   (c) when the phosphonate is selected from phosphonates of        formula (III), phosphonates of the formula        (M₂O₃P—CH₂)₂—N—(CH₂)₂—N—(CH₂PO₃M₂)₂, or phosphonates of the        formula (M₂O₃P—CH₂)₂—N—(CH₂)₂—N—(CH₂PO₃M₂)—(CH₂)₂—N—(CH₂PO₃M₂)₂,        the scale inhibiting composition does not contain a nonionic        surfactant,    -   (d) when the phosphonate is selected from phosphonates of        formula (III), or phosphonates of the formula        (M₂O₃P—CH₂)₂—N—(CH₂)₂—N—(CH₂PO₃M₂)—(CH₂)₂—N—(CH₂PO₃M₂)₂, the        amount of the phosphonate on an active acid basis is greater        than 25 ppm based on the weight of black liquor recovered from        the digester, and    -   (e) when the phosphonate is selected from the phosphonates of        formula (IV), the amount of the phosphonate on an active acid        basis is greater than 20 ppm based on the weight of black liquor        recovered from the digester.

In the practice of the method of this invention in a chemical pulpingprocess, e.g. a Kraft process, the aqueous phosphonate composition ofthe invention is admixed with the black liquor recovered from thedigester. The aqueous phosphonate composition of the invention can beadded to the black liquor using any conventional means known to those ofordinary skill in the art. In addition, the aqueous phosphonatecomposition of the invention can be added directly to the black liquorprior to the black liquor recovery stage, i.e. prior to the black liquorevaporator, or it can be added to the black liquor during the blackliquor recovery stage, e.g. between effects of the MEE. A typicaltemperature range in the black liquor evaporator is generally in therange of about 80 to about 180° C., depending on the effect. The pH ofthe black liquor in an alkaline chemical pulping process is at least 9.In the case of a Kraft process, the pH of the black liquor is typicallyabout 10 to about 14, and more typically about 12 to about 14. Theaqueous phosphonate composition of the invention can be added to theblack liquor in any conventional manner known to one of ordinary skillin the art. For example, in a batch digester operation, the addition ofthe aqueous phosphonate composition of the invention can be a bulkaddition at the beginning of the black liquor recovery stage or duringthe black liquor evaporator cycle, or it can be added in multiplecharges throughout the black liquor evaporator cycle, or continuously asthe black liquor is recovered. It is currently preferred to add theaqueous phosphonate composition of the invention as a bulk charge at ornear the beginning of the black liquor evaporator cycle. In the case ofa continuous digester operation, the aqueous phosphonate composition ofthe invention can be added continuously to the black liquor to maintainthe effective concentration of phosphonate in the black liquor or, ifthe black liquor is held in a storage vessel prior to the black liquorevaporator stage, it can be added as described above.

The amount of a scale inhibiting composition of this invention employedis an effective amount which is that amount that is sufficient toprovide an effective scale inhibiting concentration of phosphonate inthe black liquor evaporator over time at which the formation, depositionand adherence of calcium salt scale, particularly calcium carbonate orcalcium sulfate scale, is satisfactorily inhibited in the black liquorrecovery area, and additionally in the digester and/or brown stockwashers. One of ordinary skill in the art using this invention will knowthe acceptable level of calcium salt scale in the digester, brown stockwashing area, and black liquor recovery area of the particular chemicalpulping facility, and will be able to readily select an appropriatephosphonate and concentration for addition to the black liquor toachieve the desired scale inhibition for the required time based on thedisclosure of this specification. It will be apparent to those of skillin the art after reading this specification that many factors of thetype which have been mentioned herein and others, will determine theamount of the phosphonate of the invention needed to achieve the desiredinhibition. The determination of these amounts is within the ordinaryskill of the artisan in this field without undue experimentationconsidering the direction provided herein.

A third embodiment of the invention relates to a method for inhibitingcalcium salt scale formation in an aqueous system in a selected alkalinechemical pulping process comprising (a) determining the calcium saltscale inhibition profiles of phosphonate concentration and processtemperature as a function of time for phosphonate compositions admixedwith the black liquor composition recovered from the digester of thechemical pulping process, (b) identifying the calcium salt scaleinhibition capability required by the selected chemical pulping processbased on the process operating conditions of time, temperature andpressure, and the black liquor composition, (c) selecting theappropriate phosphonate composition and phosphonate use concentration toeffectively inhibit calcium salt scale formation in the selectedchemical pulping process when the phosphonate is admixed with the blackliquor composition recovered from the digester of the selected alkalinechemical pulping process based on steps (a) and (b), and (d) admixingthe selected phosphonate composition with the black liquor compositionin the selected alkaline chemical pulping process during the blackliquor recovery stage of the chemical pulping process; wherein theselected phosphonate composition is as defined above for this invention.

A fourth embodiment of the invention relates to a method for inhibitingcalcium salt scale formation in an aqueous system in a selected alkalinechemical pulping process comprising (a) identifying the calcium saltscale inhibition capability required by the selected chemical pulpingprocess based on the process operating conditions of time, temperatureand pressure, and the black liquor composition, (b) selecting theappropriate phosphonate composition and phosphonate use concentration toeffectively inhibit calcium salt scale formation in the selectedalkaline chemical pulping process when the phosphonate is admixed withthe black liquor composition recovered from the digester in the selectedalkaline chemical pulping process based on step (a) and the calcium saltscale inhibition profiles of phosphonate concentration and processtemperature as a function of time for phosphonate compositions admixedwith the black liquor composition recovered from the digester in achemical pulping process, and (c) admixing the selected phosphonatecomposition with the black liquor composition recovered from thedigester in the selected alkaline chemical pulping process during thedigestion stage of the chemical pulping process; wherein the selectedphosphonate composition is as defined above for this invention.

In the third and fourth embodiments of the invention, the calcium saltscale inhibition profiles of phosphonate concentration and processtemperature as a function of time for phosphonate compositions admixedwith the black liquor composition recovered from the chemical pulpingprocess digester can be determined by conducting laboratory experiments,such as described herein, or by conducting larger scale testing. As eachchemical pulping process will vary depending on the type of wood beingprocessed, the specific operating conditions used, the black liquorcomposition, the composition in the digester, and the like, the specificphosphonate or phosphonate blend and the required use concentration ofsame necessary to achieve the desired scale inhibition will be dependentupon the specific chemical pulping process. By utilizing the calciumsalt scale inhibition profiles in conjunction with the calcium saltscale inhibition capability required by the selected chemical pulpingprocess based on its process operating conditions of time, temperatureand pressure, the black liquor composition, and the aqueous digestercomposition, one of ordinary skill in the art may select the appropriatephosphonate composition and phosphonate use concentration to effectivelyinhibit calcium salt scale formation in the selected chemical pulpingprocess when the phosphonate is admixed with the black liquorcomposition in the selected chemical pulping process.

The invention is further described in the following Examples which arenot intended to limit or restrict the invention. Unless otherwiseindicated all quantities and percents are expressed in weight.

EXAMPLES

A calcium salt scale test of black liquor obtained from a Kraft pulpmill in the upper mid-western United States was employed in thefollowing examples which follow and illustrate the use of thecompositions of this invention in the process of this invention. Incarrying out these tests, samples were taken of a composition of theblack liquor at selected times during the test. The concentration oftotal calcium and inhibited calcium were determined analytically usingAtomic Absorption Spectroscopy (AA). Inhibited calcium is the amount ofcalcium able to pass through a 0.45 μm filter. The general proceduredescribed below was followed. Additionally, the tests were generallycarried out at the selected inhibitor level of 100 parts per million(ppm) active acid based on the amount of black liquor, for eachphosphonate composition tested, and without inhibitor present.

BLACK LIQUOR TEST

The Black Liquor Test used herein was developed to gauge the performanceof calcium salt scale inhibition of compositions of this invention in ablack liquor composition. The black liquor composition temperature wasramped from ambient temperature to 150° C. in about 45 minutes to onehour and then maintained at 150° C. for an additional one to two hours.Samples were taken from the condenser line of the Parr Bomb Reactorusing a liquid cooled extractor at various time intervals under pressureand temperature during the test to monitor calcium concentrations usinga procedure for determination of atomic absorption outlined below.

Procedure for Charging the Parr Bomb Reactor and Test Conditions

A weak black liquor sample (about 15 wt. % solids) was taken from asample point prior to the black liquor evaporators in the Kraft processdescribed above.

The charge of phosphonate employed is on an active acid basis based uponthe weight of black liquor charged to the Parr Bomb Reactor. As usedherein, the level on an active acid basis is the amount (ppm) of purefree acid that is the molar equivalent of the actual dose of thespecific phosphonate(s) used.

Preparation of Black Liquor Sample

Approximately 1.5 L of the weak black liquor obtained above wastransferred to a 2 L volumetric flask.

In the control run, no inhibitor was added to the black liquor. In theinventive runs, enough inhibitor was added to the contents of the flaskto reach the desired concentration in 2 L, and weak black liquor wasadded to fill to the 2 L mark.

Charge of the Parr Bomb Reactor and Monitoring of Calcium Release Test

Black liquor was prepared according to the above procedure.

Prior to running each test, the Parr Bomb Reactor was acid cleaned usinga 10% sulfuric acid solution to remove any existing deposits. After theacid cleaning, the digester was rinsed with deionized water.

Black liquor with or without inhibitor (1.5 L) was transferred to theParr Bomb Reactor (2 L) and the initial temperature recorded. Theextractor line was purged with nitrogen and a 5 mL sample was taken forAA analysis. The heating sequence was initiated and time recorded ast=0. The heating sequence was to heat the contents of the Parr BombReactor from room temperature to 150° C. in 1 hour and to hold at 150°C. for the remainder of the test (approx. 1-2 hours).

(The AA analysis is done by atomic absorption by flame photometry usinga Perkin-Elmer Model 100 spectrometer; see generally, InstrumentalMethods of Analysis, Hobart H. Willard, Lynn L. Merritt, Jr.; John ADean, 4^(th) Edition, D. Van Nostrand Company, Inc. August 1965)

Quantitatively one milliliter (mL) of the sample was transferred to acentrifuge tube with 5 mL of 4% HCl solution and AA was used todetermine the calcium content of the sample, i.e. Total Calcium. Theremaining sample was drawn into a 10 mL syringe and filtered through a0.45-μm syringe filter. Quantitatively one mL of the filtrate wastransferred to a centrifuge tube with 5 mL of 4% HCl solution and AA wasused to determine the calcium content of the filtrate, i.e. InhibitedCalcium.

Every 10 minutes for the length of the test, e.g. approximately 2-3hours, the liquor in the condenser line was purged, a temperaturemeasurement was made, and approx. a 5 mL liquor sample was pulled. TheAA analysis procedure as described above was then repeated. At the endof the test, the calcium content and temperature data were plottedversus time.

Each example below was carried out according to the general procedurecited above. All levels are given in parts per million phosphonate on anactive basis by weight of black liquor.

The phosphonates used in the examples were obtained from Solutia Inc.(St. Louis, Mo.). Except as specified herein, chemicals used in theexamples were obtained from Fisher Scientific.

Tables 3-7 hereinafter following provide the data for a series of testruns performed on the black liquor using various phosphonates. Thephosphonate tested if identified by product name (as defined in Tables 1and 2 herein) in the header of each Table below. The temperature is indegrees Celsius. Parts per million (ppm) of calcium is in parts permillion by weight based on the liquor.

Example 1

A black liquor sample with no inhibitor added (Control) was tested inthe test described in the Examples section. The results are given inTable 3 below.

TABLE 3 Control—No Inhibitor Time, Minutes Total Calcium, ppm InhibitedCalcium, ppm Temp., ° C. 0 34.4 28 19 10 34.6 27 23 20 35 28.1 55 3036.3 27.7 78 40 33.7 28.6 99 50 35.6 28.4 121 60 34.1 21.7 140 75 32 4.5151 90 30.8 2.6 150 105 31.5 1.6 150 120 29.8 0.6 150

Example 2

Dequest 2006 was tested in the test described in the Examples section at100 ppm active acid. The results are given in Table 4 below.

TABLE 4 100 ppm Dequest 2006 Time, Minutes Total Calcium, ppm InhibitedCalcium, ppm Temp., ° C. 0 25 22.6 19 30 25.2 24.5 103 40 25.1 23.6 13450 26.3 22.5 150 60 26.3 22.8 150 70 26.3 22.1 150 80 26.2 19.7 150 9026.3 18.2 150 105 26.2 12.6 150 120 23.6 8.6 150

The data of this example demonstrates that a use level of 100 ppmprovided significant improvement in calcium inhibition compared to noinhibitor. The data of this example suggests that Dequest 2000 andDequest 2006 would be effective to inhibit calcium salt scale accordingto the invention.

Example 3

Dequest 2016 was tested in the test described in the Examples section at100 ppm active acid. The results are given in Table 5 below.

TABLE 5 100 ppm Dequest 2016 Time, Minutes Total Calcium, ppm InhibitedCalcium, ppm Temp., ° C. 0 34.2 28.9 20 10 34.9 27.5 37 20 34.7 29.2 7930 36.8 27.7 120 40 36.2 28.6 148 50 37.2 26.3 150 60 35.5 25.4 149 7036 25.4 150 80 34.4 25.6 150 90 34.9 24.4 150 105 36.2 23.6 150 120 33.922.9 150

The data of this example demonstrates that a use level of 100 ppmprovided significant improvement in calcium inhibition compared to noinhibitor. The data of this example suggests that Dequest 2010 andDequest 2016 would be effective to inhibit calcium salt scale accordingto the invention.

Example 4

Dequest 2066 was tested in the test described in the Examples section at10 ppm and 100 ppm active acid. The results are given in Tables 6 and 7below.

TABLE 6 100 ppm Dequest 2066 Time, Minutes Total Calcium, ppm InhibitedCalcium, ppm Temp., ° C. 0 25.7 25.8 19 10 26.2 25.9 31 20 27.1 26.1 6930 26.3 25.2 112 40 26.9 22.4 134 50 24.5 23 150 60 26 22.7 150 70 26.923.1 150 80 27.1 23.5 150 90 23.5 22.5 150 105 26.1 23.1 150 120 26 22.8150

TABLE 7 10 ppm Dequest 2066 Time, Minutes Total Calcium, ppm InhibitedCalcium, ppm Temp., ° C. 0 22.5 21.5 49 15 23.1 21.3 101 30 23 21.5 13240 23.5 22.1 150 50 23.3 21.2 150 60 22.9 18.5 150 70 22.9 14.4 150 8023.2 9.4 150 90 22.8 6.5 150 105 23.2 4.2 150 120 22.4 2.8 150

The data of this example demonstrates that a use level of 100 ppmprovided significant improvement in calcium inhibition compared to theuse of no inhibitor or 10 ppm inhibitor. The data of this examplesuggests that a use level of about 40 ppm to about 500 ppm for Dequest2060 and Dequest 2066 would be effective to inhibit calcium salt scaleaccording to the invention.

The preceding description is for illustration and should not be taken aslimiting. Various modifications and alterations will be readilysuggested to persons skilled in the art. It is intended, therefore, thatthe foregoing be considered as exemplary only and that the scope of theinvention be ascertained from the following claims. It is furtherintended that each and every claim limitation be literally construed toinclude any and all variants which are insubstantially different fromwhat is literally recited except variants which are in the prior art.

1. A scale inhibiting composition for inhibiting calcium salt scaleformation in alkaline aqueous mixtures of chemical pulping processes,said composition comprising the black liquor of said chemical pulpingprocess and an effective scale inhibiting amount of at least onephosphonate selected from compounds having the formula:M₂O₃P—CH₂—N(R¹)—(CH₂)_(m)—N(R²)—CH₂PO₃M₂  (I), compounds having theformula:R³—C(OH)(PO₃M₂)₂  (II), compounds having the formula:N—(CH₂PO₃M₂)₃  (III), phosphonates having the formula:

amine oxides of phosphonates of formulas (I) and (III), or mixturesthereof; wherein M is independently selected from hydrogen, alkalimetal, alkaline earth metal or ammonium, R¹ and R² are independentlyselected from —CH₂PO₃M₂ or —(CH₂)_(n)—N—(CH₂PO₃M₂)₂, m is 2 or 3, n is 2or 3, and R³ is an alkyl group having 1 to 17 carbon atoms and R³ isoptionally branched and optionally unsaturated; with the provisos that:(a) said phosphonate is not a blend of a phosphonate of formula (II)with a phosphonate of formula (III), (b) said phosphonate is not a blendof a phosphonate of formula (II) with a phosphonate of the formula(M₂O₃P—CH₂)₂—N—(CH₂)₂—N—(CH₂PO₃M₂)₂, (c) when said phosphonate isselected from phosphonates of formula (III), phosphonates of the formula(M₂O₃P—CH₂)₂—N—(CH₂)₂—N—(CH₂PO₃M₂)₂, or phosphonates of the formula(M₂O₃P—CH₂)₂—N—(CH₂)₂—N—(CH₂PO₃M₂)—(CH₂PO₃M₂)₂, said scale inhibitingcomposition does not contain a nonionic surfactant, (d) when saidphosphonate is selected from phosphonates of formula (III), orphosphonates of the formula(M₂O₃P—CH₂)₂—N—(CH₂)₂—N—(CH₂PO₃M₂)—(CH₂PO₃M₂)₂, the amount of saidphosphonate on an active acid basis is greater than 25 ppm based on theweight of black liquor recovered from the digester, and (e) when thephosphonate is selected from the phosphonates of formula (IV), theamount of the phosphonate on an active acid basis is greater than 20 ppmbased on the weight of black liquor recovered from the digester.
 2. Thecomposition of claim 1 wherein M is independently selected from hydrogenor an alkali metal.
 3. The composition of claim 2 wherein M is sodium orpotassium when M is an alkali metal.
 4. The composition of claim 1wherein R¹ and R² are CH₂PO₃M₂.
 5. The composition of claim 4 wherein mis
 2. 6. The composition of claim 1 wherein R¹ and R² are(CH₂)_(n)—N—(CH₂PO₃M₂)₂.
 7. The composition of claim 6 wherein m is 2and n is
 3. 8. The composition of claim 1 wherein R¹ is CH₂PO₃M₂ and R²is (CH₂)_(n)—N—(CH₂PO₃M₂)₂.
 9. The composition of claim 8 wherein m is 2and n is
 2. 10. The composition of claim 1 wherein R³ is an alkyl grouphaving 1 to 5 carbon atoms.
 11. The composition of claim 10 wherein R³is methyl.
 12. The composition of claim 1 wherein said phosphonate is atleast one phosphonate of formula (I).
 13. The composition of claim 1wherein said phosphonate is a mixture of at least two phosphonates offormula (I).
 14. The composition of claim 12 wherein said phosphonate is(M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)₂ and the amount of said phosphonate on anactive acid basis is about 10 ppm to about 1000 ppm based on the weightof black liquor recovered from said digester.
 15. The composition ofclaim 12 wherein said phosphonate is(M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)₂ and the amount of saidphosphonate on an active acid basis is about 30 ppm to about 1000 ppmbased on the weight of black liquor recovered from said digester. 16.The composition of claim 12 wherein said phosphonate is(M₂O₃PCH₂)₂NCH₂CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂CH₂N—(CH₂PO₃M₂)₂and the amount of said phosphonate on an active acid basis is about 10ppm to about 1000 ppm based on the weight of black liquor recovered fromsaid digester.
 17. The composition of claim 13 wherein said phosphonateis a mixture of:(M₂O₃PCH₂)₂NCH₂CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)CH₂CH₂CH₂—N(CH₂PO₃M₂)₂,and a second phosphonate selected from N(CH₂PO₃M₂)₃,(M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)₂, or(M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂.
 18. The composition ofclaim 17 wherein said second phosphonate is N(CH₂PO₃M₂)₃, and the amountof said mixture on an active acid basis is about 10 ppm to about 1000ppm based on the weight of black liquor recovered from said digester.19. The composition of claim 17 wherein said second phosphonate isselected from (M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)₂, and the amount of saidmixture on an active acid basis is about 20 ppm to about 1000 ppm basedon the weight of black liquor recovered from said digester.
 20. Thecomposition of claim 17 wherein said second phosphonate is selected from(M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)₂, and the amount of saidmixture on an active acid basis is about 10 ppm to about 1000 ppm basedon the weight of black liquor recovered from said digester.
 21. Thecomposition of claim 13 wherein said phosphonate is a mixture of(M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)₂ and(M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)CH₂CH₂N(CH₂PO₃M₂)₂, and the amount of saidmixture on an active acid basis is about 20 ppm to about 1000 ppm basedon the weight of black liquor recovered from said digester.
 22. Thecomposition of claim 1 wherein said phosphonate is a mixture of(M₂O₃PCH₂)₂NCH₂CH₂N(CH₂PO₃M₂)₂ and N(CH₂PO₃M₂)₃, and the amount of saidmixture on an active acid basis is about 30 ppm to about 1000 ppm basedon the weight of black liquor recovered from said digester.